BIOMECHANICAL STUDY OF LUMBAR PEDICLE SCREWS - DOES CONVERGENCE AFFECT AXIAL PULLOUT STRENGTH

We tested the hypothesis that two pedicle screws placed in convergence offer more resistance to axial pullout than do two pedicle screws pla ced in parallel. Eight fresh cadaveric lumbar spines, L2-L5, were harv ested. Individual vertebra were then paired for testing. Into each L2 and L3 vertebra, a pair of pedicle screws were inserted. The screws we re placed parallel (i.e., at 0 degrees convergence) in L2 and at 30 de grees convergence in L3, in the first pair. In the second pair of L2 a nd L3, this order was reversed. Alternating the convergence angle was carried out on each successive pair of L2 and L3. A section of standar d longitudinal rod was attached to each pedicle screw by using top-loa ding three-point shear clamp fixation with tangential lock screws. Tra nsverse connectors were attached superiorly and inferiorly to the long itudinal rods. As in L2 and L3, each LA and L5 received a pair of scre ws placed at either 0 or 30 degrees convergence, with the angle of con vergence alternated on each successive pair. Each pair of screws in ea ch vertebra was tested in axial pullout at 1 mm/s. Fourteen pairs of v ertebra were tested and two findings emerged: paired pedicle screws at 30 degrees of convergence offered more resistance to axial pullout (2 8.6% on average) and sustained higher loads at the clinical threshold of loosening (101% on average) than paired pedicle screws placed in pa rallel. Thus there seems to be an advantage to screws placed in 30 deg rees of convergence as compared with screws in parallel.